This invention relates to the art of blow molding articles of organic plastic material from a parison and has as its principal object the provision of improved hollow articles in an economical, expeditious manner.
The art teaches various methods and apparatus for obtaining blow molded articles of organic plastic material from a parison, such as in U.S. Pat. No. 3,349,155 and U.S. Pat. Re. No. 27,104. Generally, these methods are characterized by forming a parison in a parison mold on a blow core, placing said formed parison and blow core into a blow mold and expanding said parison in the blow mold by means of fluid pressure. After blowing the parison into the form of the hollow article in the blow mold, the blow core is separated from the article and the blow mold with the blown article therein is transferred to a discharge station.
Certain disadvantages exist in the practice of the foregoing methods. The finally expanded articles, particularly those with walls of substantial thickness, tend to require support during an extended cooling period before they can be safely discharged from the mold. If the cooling step is inadequate, the articles will tend to deform. On the other hand, the cooling time of thick-walled articles tends to be long, and the blow mold would therefore be engaged during such a cooling step, rather than available for participation in the expansion of another article.
In other instances, for example, when molding polyesters, the expansion is preferably carried out in more than one step, each step occurring at different temperatures, which introduces an additional time interval during which the blow mold is unavailable to produce another aticle.